The present invention relates generally to photogrammetry, and more particularly, to improved low temperature photogrammetry for satellite systems.
Historically, photogrammetry of test objects has been performed in a thermal test chamber that incorporates a transparent window into an outer test chamber wall. The transparent window acts as a barrier between the measurement equipment and the test environment, while providing visual access for the photogrammetric equipment through the window. At low temperatures it has been necessary to heat the outer surface of the window to prevent the formation of condensation and consequent loss of visibility. As a result of this heating, a thermal gradient exists across the thickness of the window. Historical data suggests that the thermal gradient causes both a physical and optical distortion in the window, which negatively affects the accuracy of photogrammetry measurements (data) acquired through it. The disadvantage of the current system is that measurement accuracy is degraded at temperatures approaching or equaling those experienced by satellite components in space.
In order to improve low temperature (i.e. below condensing or frosting temperature) photogrammetry, the optical distortions should be limited or eliminated. This can be accomplished by limiting or eliminating the thermal gradient that exists across the visual access window or by removing the transparent window entirely.
It is therefore an object of the present invention to limit or eliminate thermal gradients induced upon photogrammetry system windows by moving the window from the outer thermal wall to a position inside the housing containing the photogrammetry camera. A gate valve is added between the outer thermal wall and the window to act as a barrier to the outside environment. The gate valve opens for a limited time to allow individual photogrammetry exposures, and closes before a significant thermal gradient can be induced upon the transparent photogrammetry window. To assist in the thermal isolation of the transparent photogrammetry window within the housing, pressurized dry gas (typically nitrogen) at ambient temperature and compatible with the housing environment is introduced into the housing. The gas maintains the transparent photogrammetry window near ambient temperatures when the gate valve is closed and acts as a thermal barrier between the transparent photogrammetry window and the thermal test chamber when the gate valve is open.
It is another object of the present invention in an alternative embodiment to eliminate the use of the transparent photogrammetry window entirely by replacing it with a gate valve. The gate valve is opened for a limited time to allow for photogrammetry exposures. The gate valve remains closed at all other times to maintain the photogrammetry camera near ambient temperatures. To assist in the thermal isolation of the camera, pressurized dry gas is introduced in the housing, thus preventing the formation of condensation on the camera body or lens. In addition, while the gate valve is open, additional pressurized dry gas can be introduced near the camera and lens to act as an additional thermal barrier.
Because of these features the present invention is particularly suitable for making photogrammetric measurements of satellite components on earth in simulated extreme temperature conditions. In this way, satellite components may be evaluated for such things as thermal stability prior to being placed in space.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.